Projection screen



Dec. 1, 1953 c. G. BURTON 2,660,927

PROJECTION SCREEN Filed Dec. 12, 1949 Patented Dec. 1, 1953 UNITEDSTATES ATENT OFFICE 2 Claims.

My invention relates to a projection screen, and more particularly to aprojection screen of the reflecting or front-projection type for thepurpose of projecting motion pictures, slide films, television images,and the like.

The principal objection to front-projection screens now available isthat they require a darkened room for the showing of pictures. Whilethis is not a very serious handicap in a public motion picture theaterwhere darkness and air-conditioning are customarily present, it becomesa serious handicap in the many instances where pictures are shown indaytime to more or less crowded groups such as school and othereducational classes, club groups and the like.

The principal reasons that presently available screens cannot be used ina lighted room are that they absorb some of the projected light, theyreflect the projected picture in all directions from the screen, thuswasting a large part of the projected light by sending it up and down aswell as toward the audience, and they collect and diffuse in alldirections light coming from sources other than the projector, thusdestroying the necessary contrast in the picture. This necessitates anartificial darkening of the room with consequent loss of ventilation,eyestrain, and inability of the audience to move around freely or totake notes. The process of artificially darkening a room is also quiteexpensive.

My invention consists of a screen which makes it possible to view clear,bright images from all parts of a normally lighted room in daytime. Thisis accomplished by using a clear piece of transparent material, such asglass, the back of which is formed into closely spaced ribs whose crosssection approaches a sinusoidal curve, said ribs running vertically ofthe screen. These ribs are mirror-plated. The front surface is roughenedsufiiciently to remove the glaze and to form a refracting surface withvery low reflective power. I have found that one way to produce thisrefracting surface is to sandblast said surface until all glaze isremoved, and then to make it more nearly transparent by applying one ormore very thin coats of varnish or other transparent film.

In the drawings:

Figure 1 is a perspective view of a portion of my screen.

Figure 2 is a cross section taken on the line 2-2 of Figure 1.

Figure 3 is a fragmentary section similar to Figure 2 showing theapproximate paths of light rays entering the screen at various points.

A screen constructed in accordance with my invention has a base plate Iof glass or other transparent material, forming a large square orrectangular viewing screen having a front surface 2 and a rear surface3.

The front surface 2, which is the viewing surface, is treated to causeit to slightly refract light projected upon it. In the preferred form ofmy invention the front surface 2 is substantially flat but roughenedslightly by sandblasting and it is then given a coat of varnish or othertransparent film. This coating is extremely thin and in the preferredembodiment of my invention the coating of varnish 4 upon the roughenedsurface 2 is formed by painting or spraying and then removing as much aspossible with a dry cloth before the varnish sets.

The rear surface 3 of the screen is formed to have a sinusoidal curvedsurface to form ribs 5 running vertically of the screen. These ribs arethen given a mirror coating 6.

The operation of my screen is shown in Figure 3. Beams of light 1 and 8from the projector enter the front surface 2 where they are slightly Irefracted as shown by the ray lines 9, ii! and ll,

12, respectively. This light passes through the plate I of transparentmaterial and strikes the mirrored surface 3. The light is then reflectedback toward the front surface of the screen where it is refracted stillmore and emerges from the screen to reach the eyes of the audience. Thebeams of light it and If! strike a portion of a rib 5 at a point wherethe rib is substantially parallel to the front surface of the screen andwill be reflected back toward the projector as shown by the lines is andMl. The beams of light 5! and I2,

striking the rib 5 on the curved surface at an angle to the frontsurface 2, will be reflected largely to one side as shown by the lightlines 55 and IE and will be further refracted in the same direction asthey leave the front surface of the screen as shown by the lines I I,Ill, I9 and 20.

Very little of the projected beam, however, is reflected or refracted toany point above or below the screen as the sinusoidal ribbed surface hasthe ribs only running in a vertical plane. Thus, the projected lightcoming from the screen is confined to an area normally occupied by theaudience and is not wasted in an up and down direction as inconventional screens.

The smallest detail that is visible in any projected image that isviewed on this screen must be wider than any single rib; thus eachdetail is reflected to every part of the audience. Therefore, thedistance between ribs regulates the smallest image that can be seen frommy screen.

I have found that the number of ribs 5 required per inch of screen widthis determined by the minimum viewing distance of the audience. Wheresome members of the audience may be as 5 close as four feet to thescreen, fifteen or more ribs 5 to the inch are required. At a minimumdistance of twenty feet, three ribs 5 or more per inch are required,while on larger screens where the audience is at a greater minimumdistance from the screen, proportionally fewer ribs 5 per inch arerequired.

With my screen a brilliant image may be viewed anywhere within an arc of190 around. the front of the screen. Also, distortion of the image whenviewed from one side or the other s reduced greatly as compared todistortion of conventional screens.

While it is necessary that the ribs 5 be of a uniform shape on any onescreen to avoid stre or distortion of the ima es, it possible o thisshape for specific e curve on one side e Lbs will result n t. r i ingthe image to one I have found that est brilliance of the image withoutviewing angle to less than 160, r; depth of the ribs to the width of ribmust be approximately 1 to line. the depth of the rib in proportion to twill result in a less brilliant i the depth of the rib in prop willresult in a brighter image but xvi. a area of brilliance to a smallerarc in front 0 the screen.

The thickness of the plate i is also ii... since too great a thicknesswill result in l, Jo. refraction of each .ndividual beam through thefront of the glass, thus ulting in.

a fuzzy image. It is a well known fact in optics 40 that there is nosuch thing as a truly sharp definition in a projected image, but thelimitations of the human eye make this unnecessary. I have found thatwhere the minimum viewing distance is four feet, apparent sharpness ofthe image is obtained on the transparent plate l of not overthree-sixtecnths of an inch in thickness, while at a minimum viewingdistance of twenty feet not over one'loui'th of an inch of thickness maybe used, while at greater minimum viewing dis- 5O tances a thickermaterial may be Use of the ibbed reflecting surface 3 without the frontrefracting surface 23 t. not produce a complete image since it does notreflect light from every part of the screen to each individual in theaudience. Use of the refracting surface 2 alone results in a dim imagewith little contrast, not suitable for showing in a normally lighted 4room. Use of the refracting surface 2 with a planar reflecting surface 3results in reflecting a bright image to the area immediately around theprojector, but does not give a wide enough viewing angle to be ofpractical use.

While I have described the preferred embodiments of my invention, it isto be understood that I do not wish to be limited to the details ereinset forth, but my invention is of the full scope of the appended claims.

I claim:

1. In a viewing screen, a sheet of transparent material having a frontsurface and a rear surface, said front surface being substantially fiat,said front surface being slightly and uniformly roughened and having athin coat of transparent material thereon to render the front surfacetransluscent, said rear surface having parallel regularly spacedvertical ribs, each rib being continuous and of uniform cross-sectionalshape from top to bottom of the screen, said ribs being shaped to formsinusoidal curves, said rear surface having a reflective coating,whereby substantially all light entering said screen is reflectedforwardly and horizontally laterally of the screen and loss byreflection upwardly and downwardly is minimized.

In a viewing screen, a sheet of glass having a front surface and a rearsurface, said front surface being substantially flat, said frontsurfac-v having a rough surface of the type produced by lightsandblasting and having a coat of varnish thereon to render the frontsurface translusccnt, the roughened surface having myriad high and lowpoints, and said coat of varnish b ing extremely thin on said highpoints and. slightly thicker at said low points, said rear surfacehaving parallel regularly spaced vertical ribs, each rib beingcontinuous and of uniform cross-sectional shape from top to bot tom ofthe screen, said ribs being shaped to form sinusoidal curves, said rearsurface having a reflective coating, whereby substantially all lightentering said screen is reflected forwardly and horizontally laterallyof the screen and loss by reflection upwardly and downwardly isminimized.

CHARLES G. BURTON.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 1,044,135 Buechner Nov. 12, 1912 1,491,830 Troeger Apr. 29,1924 1,535,985 Clark Apr. 28, 1925 2,086,556 Jacobson July 13, 19372,480,031 Kellogg Aug. 23, 1949

